Estrogen loss with reproductive aging in women is a major risk factor for osteoporosis in later life. Elderly women have elevated levels of undercarboxylated osteocalcin (ucOC), which indicates vitamin (vit) K inadequacy, and this has been associated with the risk of hip fracture. ucOC is also higher following the menopause when estrogen status has declined. In animal studies estrogen inhibits the development of vit K deficiency. Little is know about the role of estrogen in the regulation of bone vitamins in women. The purpose of this study is to determine if vit K status is compromised by declining estrogen status during the perimenopausal (PM) transition. The aim will be to determine if estradiol (E2) levels are related to ucOC. Archived plasma samples from perimenopausal women (menstrual cycle day 6 of the follicular phase), aged 45 to 54, X BMI 24 plus/minus.6, who have low E2 (less than 35 pg/mL) or high E2 (greater than 70 pg/mL) will be examined for level of ucOC and total osteocalcin (OC). Univariate testing for differences between groups or ucOC as a percent of total OC, and independent factors (age, E2, estrone and follicle stimulating hormone) that predict ucOC as a percent of total OC will be examined. Confirming data would suggest vit K status may be compromised at midlife, rather than old age. Potentially, dietary vit K intake may need to increase above current guidelines for midlife women to protect bone health.